The new fleet may not enter passenger service until 2021, operator Strathclyde Partnership for Transport (SPT) admitted.
Contractors have hit problems installing new signalling equipment in the 123-year-old underground system – the world’s third oldest.
This has been compounded by the need to retain existing equipment so the Subway can continue to operate throughout the overhaul.
An SPT report stated: “Development of the migration installation and strategies for the various control systems is under way but has experienced significant delays.
“The inherent complexity of install[ation] in 19th century tunnels and the associated required detailed management to ensure limited impacts to the operational and passenger environments has presented challenges to the completion of installation designs.”
SPT said it had expressed “significant and on-going concern” to the contractors consortium of Italian firm Ansaldo – now Hitachi – which is responsible for the signalling , and Swiss company Stadler, which is building the 17 new trains.
SPT said that had prompted the companies to replace several key staff, including the project director. “with more experienced individuals” who had “intimated their current programme is no longer achievable”.
A proposed revised timescale is due to be submitted early next year, which SPT expects will “move some key dates significantly”.
SPT director of Subway Antony Smith said: “Around 18 months to the current programme is likely.”
SPT announced when the £200 million contract was agreed three years ago: “The public should start to see the first new trains introduced to the system in 2020”.
The first three trains are being prepared on a test track near Ibrox and are expected to undergo overnight trials in the Subway next year.
Once in service, they are expected to initially operate with drivers before switching to “unattended train operation” like some underground lines in Paris, Barcelona and Copenhagen.
Other UK systems without drivers, such as the Docklands Light Railway in London, still have on-board staff to operate the doors.
The trains will increase the peak four-minute frequency to two to three minutes.